Reduction of CO₂ to methanol by a polyenzymatic system encapsulated in phospholipids-silica nanocapsules

By reversing the biological metabolic reaction pathway of three dehydrogenases, a formate dehydrogenase, a formaldehyde dehydrogenase and an alcohol dehydrogenase, it was possible to transform CO₂ into methanol by a cascade reaction. The activity of each enzyme was examined separately and then the relative amount of each enzyme for the cascade reaction was optimized. The enzymes consume one molecule of the NADH cofactor each to run which should be regenerated for cost reasons. Three different NAD⁺ regenerating systems were compared: 2 enzymes (phosphite dehydrogenase (PTDH) and glycerol dehydrogenase) and a natural photosystem extracted from spinach leaves (chloroplasts). PTDH was proven to be more efficient at neutral pH. The new polyenzymatic system (4 enzymes) was then encapsulated in silica nanocapsules (internal diameter 30 nm) nanostructured by phospholipids (NPS). This hybrid nanobioreactor showed an activity 55 times higher than the free enzymes in solution. A methanol production of 42 μmol g_NPS^-1 corresponding to 4.3 mmol g_{commercial enzymatic powder}^-1 in 3 h at room temperature and 5 bar was obtained.